Colloidal silica is widely utilized as a reinforcing agent for elastomers such as sealants, rubbers, etc., as a sedimentation inhibitor in the systems containing fillers or pigments, a thickener or a thixotropic agent of liquid resins, and a blocking inhibitor for films for the purposes of increasing the stiffness, strength and/or the dimensional stability of materials, increasing the toughness and/or the impact resistance of materials, increasing the heat strain temperature of materials, increasing the mechanical decay of materials, decreasing the permeability of materials to gases and liquids, improving the electric properties of materials, reducing the cost of materials, and the like.
When colloidal silica particles are filled in a polymer, the various properties shown by the composite material of the polymer containing the colloidal silica fine particles are decided by the characteristics of the components constituting the composite material, the dispersed state of the colloidal silica fine particles, and the interface characteristics between the colloidal silica fine particles and the matrix. It is known, in the properties described above, the morphology (the dispersed state of colloidal silica fine particles) of the composite film and the interface characteristics are important as the parameters inherent to the composite material and give large influences on the performance of the composite material.
In general, in the case of dispersing inorganic fine particles in a polymer, a method of increasing the affinity of the inorganic fine particles and the polymer matrix by treating the inorganic fine particles with a coupling agent for improving the dispersed state of the inorganic fine particles in the polymer matrix is known but there are many cases that a good dispersibility is not obtained and the physical strength is insufficient in the coupling agent treatment. Accordingly, in place of the method described above, investigations for improving the dispersibility of inorganic fine particles by coating or grafting the inorganic fine particles with a polymer have been carried out.
As the method of coating inorganic fine particles with a polymer, there are generally "a method of using a previously prepared polymer" and "a method of polymerizing a monomer on the surfaces of inorganic fine particles and coating the particles with the formed polymer".
"As the method of using a previously prepared polymer", there are a method of adsorbing or chemically bonding the previously prepared polymer to the inorganic fine particles, a coacervation method, a heteroaggregation method, a dry blend method, and the like but in these methods, there are many restrictions for optionally controlling the coating amount of the polymer on the surfaces of- the inorganic fine particles, and also when the particle sizes of the inorganic fine particles are small, the application of these method is difficult.
On the other hand, "the method of polymerizing a monomer on the surfaces of inorganic fine particles and coating the particles with the formed polymer" is an excellent method in the point capable of carrying out the formation of a polymer and coating of the inorganic fine particles with the polymer formed in one stage and also in the point of imparting functions to the coated particles such as easily introducing polar groups and/or functional groups to the coating polymer. The researches about the surface treatment of inorganic powders by such a polymerization method are described in detail in Kroker et al., Progr. Org. Coatings, 1, 23(1972) and Nagai et al, Kobunsi Kako (High molecular Processing), Vol. 39, No. 11, 537-542(1990).
Also, as to "the method of polymerizing a monomer on the surfaces of inorganic fine particles and coating the particles with the formed polymer", various methods such as a polymerization method by the active species on the surfaces of inorganic fine particles, a polymerization method by the adsorption or bonding of an initiator, a polymerization method by the adsorption, bonding, or intercalation of a monomer, a polymerization method by the solubilization of a monomer to the surface adsorption layer, a polymerization method by the precipitation and adsorption of-growth groups, and the like are proposed in K. Nollen et al., Angew. Makromol. Chem., 6 1(1069), F. Runge et al., Makromol. Chem., 81, 68(1965), K. Furusawa et al., J. Colloid Interface Sci., 109, 69(1986), M. Hasegawa et al., J. Polym. Sci., Polym. Chem. Ed., 25, 3117 and 3231(1987), etc. However, in the methods described above, there are many problems that the control of the amount of the polymer formed is difficult, a large amount of aggregation is formed, thereby it is difficult to keep the stability of the dispersion, a large amount of homopolymer particles is separately formed, the monomer species which can be applied are restricted and thus, the methods are poor in general uses, and also reaction process of many stages is required, whereby the methods are poor in the practical use.
Furthermore, a method wherein a polymerization functional group (vinyl group) is introduced by the chemical modification of the hydroxyl group on the surfaces of silica fine particles and by the copolymerization of the inorganic fine particles having the vinyl group at the surfaces of the particles and a vinyl monomer, a polymer is grafted to the surface of the particles is disclosed in M. Chaimberg et al., J. Appl. Polym. Sci., 37, 2921(1989). This method has a definite effect for obtaining the composite particles of a water-soluble polymer and silica but in the method, there are problems that the graft efficiency is very low and a large amount of polymers which are not bonded to the particles are by-produced.
Also, E. Bourgeat-Lami et al reported the result of carrying out an emulsion polymerization while adding dropwise ethyl acrylate and dodecyl sodium sulfate using silica particles modified by hydroxyethyl methacrylate as cores in the presence of a nonionic surface active agent in ACS Symposium Series 585 (1995), "Hybrid Organic-Inorganic Composites", Chapter 10 but in this case, although the synthesized composite particles are partially grafted to the silica particles, the particle form obtained is an aggregate form and uniform polymer coating is not performed while keeping the dispersibility. E. Bourgeat-Lami et al. stated that in the case of polymer coating of silica particles in an aqueous system, silica particles were liable to form aggregates and in addition to silica particles, which became the seeds, the polymer sole particles were liable to form, whereby it was very difficult to carry out the formation of uniform composite particles.
Moreover, JP-B-4-48832 (the term "JP-B" as described herein means an "examined published Japanese patent application") discloses an aqueous dispersible composition of the organic-inorganic hybrid type polymer particles obtained by emulsion-polymerizing a (meth)acrylic acid ester monomer, a monomer containing a polymerizable unsaturated double bond and an alkoxysilyl group together in the molecule and colloidal silica in an aqueous medium in the presence of an anionic surface active agent and/or a nonionic surface active agent. However, the production method- described in the above patent publication is a method of polymerizing by simultaneously adding the vinyl monomer and the monomer containing the polymerizable unsaturated double bond and the alkoxysilyl group together in the molecule to an unmodified colloidal silica dispersion and as the result of out investigations, it has been clarified that even in the method, coating by the polymer on the surfaces of the colloidal silica particles proceeds only partially and also many polymer sole particles are by-produced.
As described above, by the conventionally known methods, it has been difficult to obtain an aqueous dispersion of core/shell type composite particles having colloidal silica as the cores and having an organic polymer as the shells, which can optionally control the amount of the forming polymer, has a high dispersion stability, does not by-produce polymer sole particles, and has a high uniformity of polymer coating.